Ground Currents Research Articles

Electrical damage mechanism analysis of traction motor bearings on high-speed trains considering overvoltage impulse

ABSTRACT As the critical rotating knuckle, traction motor bearings play a vital role in transmitting torque from motors to wheels for high-speed trains, while some electrochemical corrosion signs appear in traction motor bearings. To maintain the safety of the traction system, it is essential to explore the electrical damage mechanism of traction motor bearings considering the impact brought from current or overvoltage impulses. Herein, via analysing the electrochemical corrosion evolutionary process of bearings, main influencing factors are determined that both transient overvoltage impulse and long-term grounding current bring dramatic impact on bearings. To assess overvoltage distribution on different carriages, a ‘catenary-train-substation’ traction power supply model is launched based on practically collected impedances. A ‘carriage-bogie-motor’ circuit model is built to evaluate the overvoltage propagation path, meanwhile, the 3D finite element geometry of the bogie with bearings is launched to observe the overvoltage distribution in the bearings for evaluating breakdown risk.

Research on the monitoring system for induced voltage and ground current of 27.5 kV cable sheath in railways

PurposeThe purpose of this study is to address the deficiency in safety monitoring technology for 27.5 kV high-voltage cables within the railway traction power supply by analyzing the grounding methods employed in high-speed railways and developing an effective monitoring solution.Design/methodology/approachThrough establishing a mathematical model of induced potential in the cable sheath and analyzing its influencing factors, the principle of grounding current monitoring is proposed. Furthermore, the accuracy of data collection and alarm function of the monitoring equipment were verified through laboratory simulation experiments. Finally, through practical application in the traction substation of the railway bureau on site, a large amount of data were collected to verify the stability and reliability of the monitoring system in actual environments.FindingsThe experimental results show that the designed monitoring system can effectively monitor the grounding current of high-voltage cables and respond promptly to changes in cable insulation status. The system performs excellently in terms of data collection accuracy, real-time performance and reliability of alarm functions. In addition, the on-site trial results further confirm the accuracy and reliability of the monitoring system in practical applications, providing strong technical support for the safe operation of high-speed railway traction power supply systems.Originality/valueThis study innovatively develops a 27.5kV high-voltage cable grounding current monitoring system, which provides a new technical means for evaluating the insulation status of cables by accurately measuring the grounding current. The design, experimental verification and application of this system in high-speed railway traction power supply systems have demonstrated significant academic value and practical significance, contributing innovative solutions to the field of railway power supply safety monitoring.

A novel cascaded H-bridge photovoltaic inverter with flexible arc suppression function

This paper presents a novel approach that simultaneously enables photovoltaic (PV) inversion and flexible arc suppression during single-phase grounding faults. Inverters compensate for ground currents through an arc-elimination function, while outputting a PV direct current (DC) power supply. This method effectively reduces the residual grounding current. To reduce the dependence of the arc-suppression performance on accurate compensation current-injection models, an adaptive fuzzy neural network imitating a sliding mode controller was designed. An online adaptive adjustment law for network parameters was developed, based on the Lyapunov stability theorem, to improve the robustness of the inverter to fault and connection locations. Furthermore, a new arc-suppression control exit strategy is proposed to allow a zero- sequence voltage amplitude to quickly and smoothly track a target value by controlling the nonlinear decrease in current and reducing the regulation time. Simulation results showed that the proposed method can effectively achieve fast arc suppression and reduce the fault impact current in single-phase grounding faults. Compared to other methods, the proposed method can generate a lower residual grounding current and maintain good arc-suppression performance under different transition resistances and fault locations.

Design and research of grounding current monitoring device for converter transformer core and clamp

The effective monitoring of the grounding current of the core and clamp of the converter transformer can prevent partial overheating, gassing of oil, and other abnormal conditions caused by the grounding fault of the core and clamp, ensuring the safety and stability of the power system. At present, the main detection and diagnosis method is to use the clamp current meter to detect the effective value of the grounding current. However, due to the larger capacity, higher voltage level, more complex structure and electromagnetic field distribution of the converter transformer, there is no clear standard and detection diagnosis method. Firstly, this paper analyzes the causes of grounding current and related calculations. And the design of the monitoring system for the grounding current of the converter transformer core and clamp design of the hardware system for the grounding current of the converter transformer is studied and the design process of the main circuit is given. Secondly, aiming at the problem of amplitude measurement deviation and non-integer frequency of harmonic generated by the grounding current of the core and clamp, a harmonic current detection and analysis scheme based on four Blackman-harris algorithms and the specific design process are proposed. Third, a diagnostic method based on probabilistic neural network is proposed, and a software platform is built to display and diagnose the grounding current state according to it. Finally, a prototype was manufactured and experimental study was carried out to verify the correctness of the proposed scheme.

Assessing the efficiency of measures to enhance electric power quality in variable-frequency drive for scraper conveyors

The intensive implementation of variable-frequency drive machines and installations in underground mining processes necessitates addressing several issues, with a primary focus on ensuring the quality of electric power. Elevating the energy resource of mining machines and enhancing the energy efficiency of mining operations requires maintaining the rated indicators of electric power quality in mine power distribution systems. Achieving this involves assessing the level and composition of higher harmonic components in voltage and current within power circuits equipped with variable-frequency drives (VFD). Key objectives encompass the development of a simulation model based on the equivalent diagram of the power distribution system substitution with a scraper conveyor VFD to scrutinize the spectral composition of higher harmonic components in the power circuits of the mine power distribution system (MPDS). Additionally, the study involves analyzing the impact of harmonic filters (HFs), reactors, and sine filters on the quality of electric power in the VFD system of a scraper conveyor. Further analysis extends to the spectral composition of higher harmonic components in circuits related to insulation leakage and metering circuits of the residual-current device. Practical recommendations for improving electric power quality in the VFD system of a scraper conveyor are then developed based on the research findings. The established model of a variable-frequency drive system for scraper conveyors facilitates the assessment of the effectiveness of electric power quality improvement measures. The harmonic composition of voltage and current in the mine power distribution system is determined under maximum distortion conditions and in the presence of HFs, reactors, and sine filters. Research methods are chosen to unveil the spectral composition of voltage and current in symmetrical and single-phase modes of insulation leakage, as well as in metering circuits of residual-current devices (RCDs). It is noted that the harmonic composition of leakage voltage and current is primarily influenced by the parameters of the output voltage modulated by the autonomous frequency converter inverter. Considering the high level of harmonic components in voltage and current, adjustments to RCD settings, capacitive current compensator, and the protective shunting unit are recommended for electrical safety. The study emphasizes the importance of scientifically substantiating the rated indicators of higher harmonic components for leakage circuits and further exploring the physiological effects of higher current harmonics on the human body. The feasibility of installing a harmonic filter (HF) directly on the low-voltage supply section of a scraper conveyor should be technically justified. Interestingly, the presence of HFs, reactors, and sine filters does not significantly impact the harmonic composition or the magnitudes of coefficients of the harmonic components in the phase voltage of the system concerning ground and leakage currents through insulation. However, higher harmonic components induced in leakage current circuits may pose a potential hazard, leading to a violation of magnetic compatibility and posing risks in case of contact with live parts of electrical equipment.

Defect Identification of XLPE Power Cable Using Harmonic Visualized Characteristics of Grounding Current

This paper proposes an online monitoring and defect identification method for XLPE power cables using harmonic visualization of grounding currents. Four typical defects, including thermal aging, water ingress and dampness, insulation scratch, and excessive bending, were experimentally conducted. The AC grounding currents of the cable specimens with different defects were measured during operation. By using the chaotic synchronization system, the harmonic distortion was transformed into a 2D scatter diagram with distinctive characteristics. The relationship between the defect type and the diagram features was obtained. A YOLOv5 (you only look once v5) target recognition model was then established based on the dynamic harmonics scatter diagrams for cable defect classification and identification. The results indicated that the overall shape, distribution range, density degree, and typical lines formed by scatter aggregation can reflect the defect type effectively. The proposed method greatly reduces the difficulty of data analysis and enables rapid defect identification of XLPE power cables, which is useful for improving the reliability of the power system.

Differential protection scheme for distribution network with distributed generation based on improved feature mode decomposition and derivative dynamic time warping

With the progress of communication technology, the cost of optical fiber and 5G continues to decrease, and data transmission becomes more convenient and fast, making it possible to realize differential protection of distribution network by various intelligent algorithms using signal waveforms. Aiming at the problem that the traditional relay protection device can not meet the actual demand when the single-phase ground fault occurs in the distribution network with distributed generation, this paper proposes a new differential protection scheme. The characteristic mode decomposition improved by the whale optimization algorithm is used to decompose the zero-sequence current waveform collected at both ends of the line. Based on the basic principle of current differential protection, the derivative dynamic time warping of the component with the largest fault feature can effectively solve the problem that the grounding current of the distribution network cannot meet the working requirements of the differential protection device, and ensure the safe and stable operation of the system. Finally, based on MATLAB software, the performance of this method is comprehensively evaluated by simulating different fault conditions, so as to ensure the feasibility and accuracy of this method in the case of diversified faults when the distributed generation is used as part of the power supply.

Experimental Studies of a Universal Mathematical Model of Leakage Currents and Ground Currents Distribution on the Elements of HVPTL

In Kazakhstan, the main power transmission line with a line voltage of 500 kV, geographically runs through the whole of Republic from north to south. The high-voltage power line is of strategic importance. The failure of one support leads to a halt in the technological process of transporting electricity to entire regions of the country. In order to increase reliability and reduce operating costs, various modifications of diagnostics and monitoring systems for overhead power lines are being widely introduced in the CIS countries. Currently, the introduction of a complex of diagnostics of structural elements of the 500 kV HVPL has become particularly relevant, firstly, the cost of damage in accidents has increased significantly, and secondly, due to the long service life, the reliability of the power system has decreased. For the successful implementation of the diagnostic complex, an accurate representation of the processes occurring during the operation of the supports is necessary. For this purpose, a universal model of leakage currents and spreading currents along the elements of the overhead line supports was developed.

Investigation and Analysis of Electromagnetic Interference for PWM Encoder of Urban Rail Train

Electromagnetic compatibility (EMC) is an important factor in ensuring the safe operation of the sensitive electronic equipment on urban rail trains. The pulse width modulation (PWM) encoder of an urban rail train exported from China to Brazil is sometimes affected by electromagnetic interference (EMI), which causes the train to fail to run properly. To solve this problem, the EMC test is performed on the PWM encoder to identify the coupling path and the type of interference source. The EMI coupling model and the vehicle-catenary-rail model are established by using an electromagnetic transients program (EMTP) to analyze the mechanism of interference coupling. It is shown that the unbalanced voltage of the train body caused by the backflow of the grounding current is the root cause of the interference of the PWM encoder. The maximum voltage coupled to the internal port of the PWM encoder is about 1894 V, which is sufficient to burn out the encoder. A measure to suppress the interference by installing thyristor surge suppressors (TSS) P0300SC is proposed, which effectively solves the EMI problem of the PWM encoder.

Analysis and Improved Behavior of a Single-Phase Transformerless PV Inverter

Transformerless inverters have an important role in the electrical energy market. The high-efficiency and reliable inverter concept is one of the most widely used inverters in single-phase photovoltaic systems because of its high efficiency, low cost, and reduced leakage ground current. However, the leakage ground current behavior depends on the power and weather conditions, which can increase the parasitic capacitance value, thus producing an increase in the leakage ground current magnitude. In this paper, it is proposed to add a passive inductive–capacitive output filter to the inverter structure in order to reduce the dependency of the leakage ground current on the system power and weather conditions. The inductive–capacitive output filter is designed in such a way that it can provide a low impedance path for the leakage ground current, different from the ground path. The proposed system was evaluated both through simulations and experimentally in a 1 kW laboratory prototype.

Analysis of the calculation methods for shell circulating current and grounding current in 500 kV HGIS and the optimization of grounding scheme

The strong electromagnetic coupling between the wire and the shell of Hybrid Gas Insulated Switchgear (HGIS) will lead to a large induced current on the shell, which will further cause local heating of the outside shell. Moreover, the utilization of grounding wire to release this current may also lead to the problem of its heating and ground potential rise. This study is intended to further promote the calculation accuracy of the shell circulating current and grounding current of the 500 kV HGIS and also propose the proper grounding schemes. The simulation results show that the ground grid must be considered in the calculation model and the phase difference of the conductor current could be ignored when the three-phase spacing exceeded 7.5 m. Furthermore, the influences of shorting bars and ground grid on the simulation results were compared and analyzed, and the optimization of grounding scheme and corresponding suggestions were put forward. Generally, it was not recommended to erect shorting bars and auxiliary ground grids. But if the temperature rise exceeds the limit, the shorting bars can be added at the two ends of a HGIS branch and the auxiliary ground grid should be laid using materials with higher flow capacity.

Phaselet Transform-Based Digital Ground Fault Protection of Grid-Connected Photovoltaic Systems

The growing interests in utilizing Photovoltaic (PV) systems are usually faced with challenges of accurate and reliable protection of these distributed generation units. A desired protection of PV systems has to effectively and accurately detect and respond to internal faults (within the PV system) and external faults (in the host grid and/or fed loads). This paper presents and tests the performance of a digital ground fault protection (DGFP) for grid-connected PV systems. The presented DGFP detects faults based on the energy contents of high frequency sub-bands of the ground current. These energy contents are extracted using the phaselet transform that can process signals with non-stationary variations in their phases. Energy contents of the high frequency sub-bands can provide accurate, fast, and reliable detection and identification of faults experienced by a PV system operated. The phaselet transform (PHT)-based DGFP is implemented for performance testing using PV systems that are operated in the grid-connected mode, and subjected to various fault and non-fault events. Performance results demonstrate accurate, fast, and reliable detection and response to different types of fault and non-fault events. Response features of the PHT-based DGFP are complimented with minor sensitivity to the level of power production and/or type and location of faults.

Mathematical modeling and experimental research on grounding current calculation of converter transformer core

When the converter transformer core is grounded at multi-points, a fault loop will be formed and a circulation will be generated, leading to local overheating of the magnetic core and decomposition of insulating oil. Therefore, the converter transformer core will be single point grounded. As a rare internal lead of converter transformer, the ground current on the core ground lead can often reflect the running state of converter transformer. In this paper, theoretical analysis, modeling simulation and experimental research are carried out on the ground current of converter transformer at one point. Firstly, according to the structure characteristics of converter transformer, the analytical modeling of ground current is carried out. Secondly, based on the structure characteristics of oil paper insulation, the equivalent capacitance of converter transformer is calculated. Then, the analytical and finite element simulation model of converter transformer is established, and the ground current of converter transformer is calculated. Finally, the correctness of the proposed scheme is verified by measuring the grounding current of converter transformer core.

Effect of Interface Defects on the Harmonic Currents in Distribution Cable Accessories under Damp Conditions

Dampness is one of the most important factors leading to the degradation of the insulation performance of cable accessories; it may easily lead to interface defects of composite insulation. In this paper, an electromagnetic field defect model of the cable composite insulation interface is established, and the effects of the defect degree and the defect type on the magnetic flux and the harmonic current are studied. The results show that the magnetic field is uniformly distributed with a decreasing trend when the cable is in good condition, and the grounding current has no obvious harmonic component. For different defects, the magnetic field changes at the defect location are notably different; for water droplets and the water film, the magnetic field distortion is obvious at the location of the defect and sheath, and for water tree defects, the distortion of the magnetic field is significant. In addition, the degree of distortion caused by the same defect at different positions is also different, and the magnetic field contrast is not obvious at different degrees of moisture exposure. The distortion of the grounding current caused by the water tree is higher than that of the water droplet and the water film, and the characteristics of the harmonic components of the grounding current caused by different defects have obvious distinctions. The type of cable defect can be judged by the proportion of harmonic components. This work can be used as a reference for the assessment of the cable insulation defect status.

A family of single-switch wide-gain converters with low voltage stress

A new family of single-switch dc–dc converters with simple structure are introduced in this paper. Firstly, using the approach of a simple switching dual structure, a switched-capacitor–inductor (SCL) cell was presented. Then, a SCL cell can be combined with the conventional circuits (Buck, Boost, Buck–Boost, Sepic, Zeta, Cuk) to provide a wide voltage conversion ratio. Taking the derived Cuk converter as an example, the comparisons among another buck–boost converters are listed, the voltage gain of the derived converter is higher than other converters, and the voltage stress on the power device is less than other converters. Furthermore, the features of single switch, common ground, continuous input, and output current are satisfied simultaneously under the condition of using few components. Finally, the converter’s performance and the correctness of the analysis are validated with results from the experimental prototype.

Research on finite element simulation of converter transformer core under multi-point grounding fault

The converter transformer is an electrical equipment with a symmetrical structure. Due to its symmetry, the ground current should be small when the core is grounded at one point. The research and analysis of grounding currents in the converter transformer can effectively help staff to judge the working state of the converter transformer. Therefore, it is very meaningful to study the calculation problem of the grounding current under the multi-point grounding state. First of all, a homogenization modeling method considering the characteristics material of the core silicon steel sheet is proposed; secondly, on this basis, the finite element simulation model of multi-point grounding of the core of the converter transformer is established and simulated; then a multi-point grounding test was conducted using the shrinkage model of the converter transformer to verify the finite element simulation results; the final consequence show that the homogenization modeling method can effectively calculate the grounding current at multi-grounding.

A Class of Bidirectional Single-Phase Z-Source AC–AC Converter With Continuous Input Current and Reduced Component Count

In this paper, a class of single-phase Z-source ac-ac converters is proposed. Share ground and continuous input current are the common features of the proposed converters. The proposed converters have unique features that differentiate them from their counterparts. These features include the buck operation in both in-phase and out-of-phase mode, being bidirectional, the minimum component counts, the ability to boost without duty cycle limitation, and reducing total power ratting of switches is known as switching device power (SDP) parameter. Reducing SDP leads to a reduction in the voltage and current rating of the semiconductors and consequently to a reduction in converter' cost. A safe commutation strategy is applied to eliminate voltage and current spikes on the switches without snubber circuits. The experimental results verify the performance of the laboratory prototype of the proposed converters.

A Rotor Ground Fault Protection Method Based on Injection Principle for Variable Speed Pumped Storage Generator-Motor

Three-phase AC excitation is adopted in the variable speed pumped storage generator-motors, which is different from the DC excitation in traditional synchronous generators. The special rotor winding structure makes the existing rotor ground fault protection methods no longer applicable. Therefore, a rotor ground fault protection method for variable speed pumped storage generator-motor is proposed in this paper. Considering that there is no precondition for additional installation of the injection device at the rotor winding neutral point, the rotor ground fault protection scheme is proposed by connecting the injection device through current limiting resistors outside the rotor winding terminals. The key parameters of injection source, band-pass filter, and current limiting resistance are analyzed and calculated. Then, the online calculation methods of grounding resistance and grounding current are proposed based on the injection equivalent circuit. On this basis, the composite protection criterion considering grounding resistance and grounding current is constructed, and the protection outlet mode is clarified. Simulation and experiments verify the effectiveness of the proposed method.

Performance Assessment of Frequency Selective Grounding for Grid-Connected Photovoltaic Systems

Photovoltaic (PV) panels and their grid-connection circuitries have to be grounded to comply with various standards and industrial codes. Objectives of grounding a PV system include limiting ground currents and potentials, supporting the operation of protective devices therein, and preventing the accumulation of static charges on PV panels. These objectives can be translated into design constraints for an impedance to connect a PV system to the ground. This paper presents the design and performance assessment of a frequency selective grounding (FSGR) for grid-connected PV systems. The proposed FSGR is composed of a parallel <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C$</tex-math></inline-formula> circuit, which provides a resistive path for dc currents and a low capacitive impedance path for harmonic currents. The FSGR is designed and experimentally tested for a 2 kW, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3\phi$</tex-math></inline-formula> , laboratory grid-connected PV system under fault and non-fault conditions. The performance of the proposed grounding is also compared with the solid grounding under similar conditions to further assess its capabilities. Experimental results show that the FSGR can reduce ground currents, limit ground potentials, and reduce the dc leakage currents. These observed features are complimented with minor effects on the operation of protective devices in a grid connected PV system.

Analytical modeling and calculation of core grounding current in converter transformer

In recent years, the calculation of the core grounding current of the converter transformer is difficult, low precision, and time-consuming, especially when the core is grounded by multiple points. This makes it difficult to analyze the core grounding current. To improve the accuracy of the calculation and reduce the complexity of the calculation, this paper carries out analytical calculation, simulation analysis, and experimental research on the grounding current of the converter transformer core. Firstly, a calculation method of core grounding current considering the winding connection of the converter transformer is proposed. In the implementation process, the converter transformer circuit model is established and the influence of saturation characteristics is considered, and the core grounding current is calculated and analyzed. Secondly, the grounding current of the converter transformer core is calculated by the finite element method (FEM). Finally, the proposed scheme is verified by experiments. The results show that the analytical method is close to the FEM and the experimental method, which indicates that the analytical method proposed can meet the accuracy requirements of engineering in this paper, and provides a new scheme for fast and accurate calculation of the grounding current of the converter transformer core.